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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorgoodevans
    • CommentTimeSep 21st 2017
    It seems normal to lay the UFH and screed towards the end of the build. In my inexperienced head I had imagined the UFH and screed being laid early over the whole ground floor with 1 course of external coursing blocks levelled and forming a boundary. With internal block walls laid on the screed.

    Am I mad - what goes wrong if its done that way.
    • CommentAuthorJamster
    • CommentTimeSep 21st 2017
    Building Regs I think require individual room heating to be adjustable now so you'd need to work your zones and rooms out earlier on. Other more experienced heads will be along soon I would think, but you might want to also think about the need for expansion joints given the potential large area, if the blockwalls are structural walls and any subsequent impact on pipework... That kind of thing.
    • CommentTimeSep 21st 2017
    As I understand it, it's a choice. You either create the finished floor and then build the intenal walls on it, or you build the internal walls and then create the finished floor in each room. I don't fully understand the reasons for deciding either way. In my case we chose to create the finished floor in each room afterwards, since that gave us the flexibility to achieve a level floor throughout despite various different floor coverings. We used battens and chipboard though, rather than screed (I wanted to minimise wet trades).
    • CommentAuthorMarkyP
    • CommentTimeSep 21st 2017
    would be OK I think if you place your insulation under slab, and UFH in the concrete slab. No reason that cant be levelled and finished flat enough to take a finished floor without an additional screed - though you'd need to make that clear to whoever was doing the ground work that this was the intent. Screed seems more problematic to me. Trying to lay sand/cement screed when not weather tight seems like a disaster waiting to happen - rain, full sun or wind drying it out too fast, or even worse - frost. Sounds like a nightmare scenario. Not sure you'd find a screeder prepared to do it. If considering liquid anhydrite screed then it's a complete non-starter, they all require the building to be weaather tight with windows and doors in or they wont pour as they need air movement to be limited to prevent too rapid a drying during the first 48hrs.

    If I did my project again I'd go with insulation under slab, UFH in slab, and have slab poured and finished to a tolerance good enough to not need a screed. As it turned out I've got 60mm anhydrite screed - lovely flat finish and super fast to install, but not cheap and you need to wait ages for it to dry.
    • CommentAuthorMike1
    • CommentTimeSep 21st 2017
    Agree with MarkyP...
    • CommentTimeSep 21st 2017
    Posted By: MarkyPNo reason that cant be levelled and finished flat enough to take a finished floor without an additional screed - though you'd need to make that clear to whoever was doing the ground work that this was the intent.

    Indeed and most important, make sure the design allows for the necessary treatment at external door thresholds. To make level access will typically mean the slab has to be reduced in height at these points if its to form the finished floor. Also any downstairs shower will probably need quite a deep hole and maybe a slope, if a wetroom. Nothing impossible but it needs to be thought out in advance.

    We were weathertight before getting to the point of thinking about whether to screed or use a timber floor, and whether to build the internal walls first or not. But our sequencing may have been unusual.
    • CommentAuthorHollyBush
    • CommentTimeSep 22nd 2017
    We did it the way MarkyP describes. The main reason was timing - if we waited for slab to dry ~6 months and then for a traditional screed to dry ~2-3 months the whole thing would have taken much longer.

    Putting the insulation below the slab supposedly helps keep a stable temperature, but heating is less responsive - it works fine for us as we are home most of the time.

    As for the screed v in slab, I think I would either take more care getting a decent finish on the slab, or go with screed - in the end the build took longer than we planned so would have made little, if any difference.

    You do need to plan where the underfloor heating goes carefully - and I would do it again every time - we love the UFH!
    • CommentAuthorgoodevans
    • CommentTimeSep 22nd 2017 edited
    OK - Thanks all - I need to give more information.

    Insulation will be under the slab. ufh will be zoned - I know where the internal walls will be.

    The plan is to have piles, blinding, cellcore (from coredek) with additional insulation on top and at the perimeter of the beams to eventually marry up with the ewi (only the piles will breach the insulation), then RC beams and slab cast in one (200mm slab). A course of blockwork coursing blocks around the perimeter (except where doors are to allow flush internal threshold), dpm over entire area, ufh laid out and screed laid.

    Advantages with the rc suspended slab are that the engineer reckons I can use medium density blockwork walls internally wherever I like, with the internal spine load bearing wall not needing to line up with the piles and beams, giving good thermal mass, solidity and crack free plaster.

    We are in the house all the time so a slow heat will not be a problem.

    Is one solution going to be using a 75mm ready mix pour for the screed layer perhaps with plastic reinforcement in the mix. Possibly with Power floating to the top of the block-work using the blocks to set the level around the perimeter. This presumably would give a good enough finish for ceramic tiles. This 'concrete screed' would be tough enough to take the rigours of the rest of the build.

    The engineer has not commented on the need for a joint in the slab (one slab 16m x 9m - the other 12m x 9m) but it is a question I will ask. if there is no joint in the slab then perhaps I don't need one in the concrete screed above.

    If I separate the slab from the screed pour It means the 'leveling' operation is separated from the slab pour - which will have its own issues ensuring the concrete is not heaped because of the cellcore. I just feel doing it all in one is too much to get right as there is a limit on the time when the concrete is workable. It will cost another 19m3 in concrete. but the stress will be lower.The other advantage is that the dpm is located in a logical place.
    • CommentAuthorcjard
    • CommentTimeSep 22nd 2017 edited
    One thing that I've found near pointless is having zoned UFH; spent around £1000 on heatmiser kit that allows for individual control of room zones for UFH, every room its own zone, massive manifolds, wiring centres several pipe runs that are much shorter than others..

    ..for my building, turns out it was nigh on a complete waste of time. I'm still running the "temporary measure" of using the ASHP's command unit as the sole thermostat for the house, all the zone actuators lie in their box unused, I fitted all the thermostats and wired them up/paired them with the hub and it lets SWMBO check the temperatures in each room remotely but they don't control anything; the pumps run at the lowest speed all the time, and I haven't even balanced the manifold flows relevant to the spec I was given by the screeders - just set them all to 1.5 litres per minute apart from a couple of rooms that were noticeably warmer than others (small rooms; wc etc) and I dialled those down to 1 litre a minute, and the whole downstairs slab acts as one zone

    The ASHP provides water at 22 degrees for the slab, the house sits at 19 degrees throughout downstairs, for probably mid-nov to early-feb then the sun tends to keep it warmer than that anyway. I'm quite surprised just how low-tech I could have gotten away with, actually. I genuinely thought control was important, but in practice it's turned out to be not so. I'm not even sure if the upstairs UFH has ever been on, and the bedrooms sit at 17 degrees in the heating season

    I've got the facility to put a high degree of control in, sure.. but will I ever do it? Probably not.. About the only change I'm planning on making is having the ASHP's "activate secondary pump" output wired up so that it can turn the pumps off when command centre isn't calling for heat. Up to now, it's been a case of turning them off manually on their dial (they have a built in regular spinup function to ensure they don't stick) when not in heating season and running them constantly when in heating season
    • CommentAuthorcjard
    • CommentTimeSep 22nd 2017
    ps; look into self compacting concrete; higher material cost but lower labour costs. Caveat: I've never looked into drying times or whether any special treatments are required before laying a floor finish like with self levelling screeds/calcium sulphates, but calling tarmac and asking them a few questions about topflow should clear those up..
    • CommentAuthorRex
    • CommentTimeNov 6th 2017

    Interesting. I have something similar. The ufh package I bought had all the bits and pieces, thermostats for each room (zone) but no mixer valves and pumps.

    I wired the thermostats, into the control panels that work the manifolds and had it all wired into the boiler. But my plumber recommend a Viesmann weather comp boiler. The way it works, it heats the ufh directly to the required, set temp, so no mixing valves, etc is needed. But when I turned the electrics on, I found that the thermostats worked against the flow valves. If the thermostats said that no heat was necessary, then the valves closed (as they should) but the WC was saying, it is cold and the boiler was fired up.

    So I have put the clips back into the control valves to keep them fully open, turned off the thermostats and allow the WC to run everything. More of less have every room at the same temp and if a room is too warm, then reduce the flow. Unfortunately, I now have redundant thermostats decorating the walls!

    It is simple, efficient and virtually nothing to go wrong. Don't understand all these mixer valves and pumps. Why heat the ufh water to a hotter temp than is necessary and then cool it to the required temp? A waste of energy IMO.
    Posted By: RexWhy heat the ufh water to a hotter temp than is necessary and then cool it to the required temp? A waste of energy IMO.

    I would agree with you with the caveat that it depends upon your heat source. e.g. if you have a wood burning boiler you can not run them at a temperature low enough for UFH so a mixer valve is needed (and probably some sort of TS)
    • CommentAuthorgoodevans
    • CommentTimeNov 7th 2017
    I will have a gas boiler - probably with 'excess PV' immersion heating. So I will use a 210l thermal store.

    In winter the bottom of the TS will have the return flow from the UFH and should be cool (< 30C), the top hot - maybe at 55-60C. In summer the excess PV will heat the whole TS to 90C (when sunny). I am still in two minds wrt a coiled TS or a Plate heat exchanger.

    I will use a simple open vented system boiler and rig the boiler pump/flowvalve for a minimum flow to give a delta T rise of 20 degrees and use a mixer valve raise the boiler return to 40C (or put another way force TS input to 60C) to give maximum efficiency for the boiler condensation. A minimum boiler run time and overrun will also be used.

    Because the heating will be slow response I will run the heating 24 hours per day (or maybe 18) setting the UFH temp (the tricky part is finding a mixer that goes that low - ideally I would like a range from 18 to 40). in the shoulder months I may run the UFH without heat to distribute heat from the sunny side of the house - I may even lay pipes in the clay under the house to dump excess summer heat) - for the benefit of SAP/part L there will be two timers and two zones but probably will be just for show. Upstairs (room in roof) I may put the MVHR through an hydronic heat exchanger to take the chill of the air - this can be decided post build as I may not need heating upstairs.
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